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Optimisation of single bunch linacs for possible FEL upgrades

Optimisation of single bunch linacs for possible FEL upgrades . Alexej Grudiev, CERN 6/02/2014 CLIC14 workshop. Linac layout and energy ugrading. Motivation from Gerardo D’Auria CLIC13. Present machine layout E beam up to 1.5 GeV FEL-1 at 80-20 nm and FEL-2 at 20-4 nm

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Optimisation of single bunch linacs for possible FEL upgrades

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  1. Optimisation of single bunch linacs for possible FEL upgrades Alexej Grudiev, CERN 6/02/2014 CLIC14 workshop

  2. Linac layout and energy ugrading Motivation from Gerardo D’Auria CLIC13 • Present machine layout • Ebeam up to 1.5 GeV • FEL-1 at 80-20 nm and FEL-2 at 20-4 nm • Seeded schemes • Long e-beam pulse (up to 700 fs), with “fresh bunch technique” FEL-1 & FEL-2 beamlines Beam input energy ≥ 750 MeV New FEL beamline l < 1 nm • Energy upgrade • Space available for acceleration 40 m • Accelerating gradient @12 GHz60 MV/m • X-band linac energy gain 2.4 GeV • Injection energy .75 GeV • Linac output energy 3.15 GeV ~50 m available 40 m (80%) available for acceleration For short bunch (< 100 fs) and low charge (< 100pC) operation

  3. Aperture scaling and BBU Growth rate of the BBU due to wakefield kick from head to tail: * Alex Chao, “Physics of collective beam instabilities in high energy accelerators”, 1993 ** Karl Bane, “Short-range Dipole Wakefields in Accelerating structures for the NLC”, SLAC-PUB-9663, 2003

  4. Transient in a cavity -> pulse compression V Short-Circuit Boundary Condition: P0 W Pout tp Pin Pin Iin Iref tk Vin Vref Analytical expression for the pulse shape · Pout Vrad Irad

  5. Effective shunt impedance ofAcc. Structure + Pulse Compressor Effective shunt impedance of TWAS+PC ** Effective shunt impedance of TWAS ** Acceleration in TWAS for transient pulse shape from PC * = + * i.e. A. Lunin, V. Yakovlev, A. Grudiev, PRST-AB 14, 052001, (2011) ** R. B. Neal, Journal of Applied Physics, V.29, pp. 1019-1024, (1958)

  6. Effective Shunt impedance in Const Impedance (CI) AS No pulse compression With pulse compression Rs0/R Rs/R Rs/R Rs/R Rs/R τs0 τs0 = 1.2564 => Rs0 /R = 0.8145 For Q = 8128; Q0 = 180000; Qe = 20000 τs0 = 0.6078 => Rs0 /R = 3.3538 But in general it is function all 3 Qs: Q, Q0, Qe

  7. Const Gradient (CG) AS No pulse compression With pulse compression Rs/R Rs/R Rs/R Rs Rs/R If the last cell ohmic and diffraction losses are equal => minimum vg. For 12 GHz, Q=8000, lc = 10mm: τs0 = 0.96; min(vg/c) = 0.032 - very low vg at the end BUT CGAS can reach higher Rs/R than CIAS Q = 8128; Q0 = 180000; Qe = 20000 τs0 = 0.5366 => Rs0 /R = 3.328 – function Q-factors Roughly the same as for CIAS with pulse compression vg_max= vg(1+0.5366); vg_min= vg(1-0.5366) Optimum vg variation is about factor 3.3 Lowest group velocity limits the CGAS performance

  8. Undamped cell parameters for dphi=150o

  9. CIAS pulse compression optimum Q0 = 180000 – Q-factor of the pulse compressor cavity(s) tk = 1500 ns – klystron pulse length Optimum attenuation: τs0 Averaged Shunt Impedance Rs0/R Rs0/R Point from slide above Point from slide above Optimum value of Qe ~ const: ranges from 20000 for Q=6000 up to 21000 for Q=8000

  10. CIAS Effective Shunt Impedance:w/o and with pulse compression Rs0 No pulse compression Rs0 With pulse compression • As expected ~ 4 times higher effective shunt impedance with pulse compression • Optimum pulse length is ~ two times longer no pulse compression is used, still it is much shorter than the klystron total pulse length

  11. CIAS linac 40 m long, <G>=60MV/m : w/o and with PC ~# of structures per 0.8x50 MW klystron 2 -> 1/5 ~20 -> ~2 Total klystron power Klystron power per structure Optimum structure length

  12. CIAS high gradient related parameters: w/o and with PC AS Esurf(z=0,t=0) AS Sc(z=0,t=0) Typical Pulse length AS Pin(t=0) Flat pulse: 230-290 ns Above the HG limits for larger apertures Peaked pulse: 122-136 ns 60-70 ns • Assamption: • Effective pulse length for breakdowns is ~ half of the compressed pulse • Breakdown limits are very close for large a/λ and thin irises • A dedicated BDR measurements are needed for compressed pulse shape

  13. CIAS with PC: max. Lstruct < 1m Rs0 For high vg corner Shorter tp Lower Qe More Ptotal Less Pin/klyst. Lower field and power quantities

  14. CIAS and CGAS with PC, different RF phase advance, no constraints CLIC_G_undamped: τs=0.31 < τs0=0.54; Ls=0.25m; Qe=15700; Pt = 400MW H75 : τs=0.50 ~ τs0=0.54; Ls=0.75m; Qe=20200; Pt = 613MW

  15. CIAS and CGAS with PC, different RF phase advance, Ls < 1m

  16. Small aperture linac, 2.4 GeV, 40m Constant Impedance Accelerating Structure with input power coupler only Klystron Pulse compressor RF load Hybrid P C

  17. Middle aperture linac, 2.4 GeV, 40m Constant Impedance Accelerating Structure with input power coupler only Klystron Pulse compressor RF load P C Hybrid

  18. Large aperture linac, 2.4 GeV, 40m Constant Impedance Accelerating Structure with input power coupler only Klystron Pulse compressor RF load Hybrid P C

  19. FERMI energy upgrade • An analytical expression for effective shunt impedance of the CI and CG AS without and with pulse compression have been derived. • Maximizing effective shunt impedance for a given average aperture gives the optimum AS+PC design of a single bunch linac • Different constraints have been applied to find practical solutions for a FERMI energy upgrade based on the X-band 2.4 GeV, 60 MV/m linac • Closer look together with beam dynamics experts is necessary to chose the right structure

  20. Motivations from PSI

  21. X-band Energy Vernier for ATHOS Parameters specs: Required energy gain: dE = +-0.4 GeV Total length available for acceleration: Lt = 16 m If: the aim to introduce the same amount of Longitudinal Wake (W_L) as in C-band Linac3: W_L3 Then: Since W_L~L/a^2: <a_X> = <a_C>/sqrt(L3_C/Lt)=6.44mm/sqrt(104m/16m)=2.53mm => <a_X>/λ=0.101 Total power from the klystrons at 1.5us: Ptot is significantly less then on can get from one XL5 and we are far from breakdown limit. => higher dE is possible even with one XL5. For example, for 0.5 m long CIAS: 40MW => 0.53GeV or 2x40MW => 0.76GeV a/λ=0.102 98% of W_L3 L_s = 0.5m 32 CI Acc. Str. Ptot = 22MW + WG loss + op. margin Const Gradient (CG) AS require the same power ConstImpedance (CI) AS have a bit higher EM fields and Sc at the input cell a/λ=0.129 61% of W_L3 L_s = 1m 16 CI Acc. Str. Ptot = 24MW + WG loss + op. margin

  22. More motivations from PSI

  23. ARAMIS energy upgrade. • It is probably unreasonable to take 0.5 m CIAS from the previous slide since it is too short and aperture is too small (there is already enough W_L in ARAMIS line) • Taking 1m long CIAS from the previous slide: 24m long X-linacwith 3 XL5s (3x40MW) can provide energy increase: dE = 1.1GeV. In this case, we may come close to the BDR limit of 4MW/mm^2 (BDR~1e-7) so we may start to see some breakdowns at this levels ! • The above 1m long CIAS is rather close to a potential Fermi linac energy upgrade structure (middle aperture). It probably can be the same structure for both projects. • A different structure (i.e. larger aperture) is maybe a better choice. More refined specs are needed to make optimized design.

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